chore: Remove dead code from `acvm_backend_barretenberg` #1033

Job	Run time
build-linux (x86_64-unknown-linux-gnu)	6m 32s
build-linux (x86_64-unknown-linux-musl)	8m 25s
build-apple-darwin (x86_64-apple-darwin)	6m 13s
build-apple-darwin (aarch64-apple-darwin)	8m 17s
	29m 27s

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